Between the illustration and the following description its operation may be clearly understood. On a pulley S, are suspended by a rope two buckets A and B, of unequal dimensions. The smaller one B, is made heavier than A when both are empty, but lighter when they are filled. It is required to raise by them part of the water from the spring or reservoir E, into the cistern Z. As the smaller Bucket B, by its superior gravity, descends into E, (a flap valve in its bottom admitting the water), it consequently raises A into the position represented in the figure. A pipe F, then conveys water from the reservoir into A, the orifice or bore of which pipe is so proportioned, that both vessels are filled _simultaneously_. The larger bucket then preponderates, descending to O, and B at the same time rising to the upper edge of Z, when the projecting pins O O, catch against others on the lower sides of the buckets, and overturn them at the same moment. The bails or handles are attached by swivels to the sides, a little above their center of gravity. As soon as both buckets are emptied, B again preponderates, and the operation is repeated without any attendance, so long as there is water in E and the apparatus continues in order. In Moxon’s machine, the buckets were filled by two separate tubes of unequal bore; the orifices being covered by valves to prevent the escape of water while the buckets were in motion; these valves were opened and closed by means of cords attached to the buckets. The efflux through F in the figure, may easily be stopped as soon as A begins to descend, by the action of either bucket on the end of a lever attached to a valve, or by other obvious contrivances. The water discharged from A, runs to waste through a channel provided for that purpose. These machines are of limited application, since they require a fall for the descent of A, equal to the elevation to which the liquid is raised in B. They may however be modified to suit locations where a less descent only can be obtained. Thus, by connecting the rope of B to the periphery of a large wheel, while that of A is united to a smaller one on the same axis, water may be raised higher than the larger bucket falls, but the quantity raised will of course be proportionally diminished. In the face of these securing advantages it has fallen into disuse; it was much too complex and cumbersome, and of too limited application. The principle of self-action in all these machines is no modern discovery, for it was described by Hero of Alexandria, who applied it to the opening and closing the doors of a temple, and to other purposes. THE PULLEY AND WINDLASS.